Piston reciprocation in internal combustion engines often causes the engine to vibrate. More specifically, the motion of an engine's pistons within their respective cylinder bores causes vibrations because the pistons' movement is started and stopped twice during each rotation of the engine's crankshaft. The inertial effects of each piston cause primary vibrations along the axis of that piston.
One known method of damping or offsetting primary vibrations is to mount one or more counterweights to the crankshaft. The counterweights are most effective when they are spaced radially from the crankshaft axis of rotation, with the crankshaft axis of rotation intermediate the crankpin and the counterweights. Although the counterweights orbit about the crankshaft axis of rotation in a generally circular path, the counterweights move generally opposite the direction of travel of the linearly-reciprocating pistons.
While it is possible to offset most or all primary vibrations of the piston with such counterweights, movement of the counterweights causes further vibrations that may not be aligned with the primary vibrations. Thus, as the mass of the counterweights is increased to further offset the primary vibrations of the pistons, the moving counterweights create higher-order vibrations.
The vibration environment becomes even more complex in a V-twin, single-crankpin engine. In that type of engine, the pistons are interconnected with the same crankpin, and are oriented at a 45.degree. angle with respect to each other. Therefore, the pistons do not start and complete their respective strokes in unison. The resultant combined primary vibration acts generally vertically and transverse to the crankshaft axis of rotation.
It is known to provide one or more balancer shafts in combination with crankshaft-mounted counterweights to substantially offset primary vibrations. One known configuration includes a pair of balancer shafts disposed on opposite sides of the crankshaft, and rotating in timed sequence with the crankshaft.
Typically, no balancing system is perfect, and there will often exist higher order vibrations that cannot practically be dampened. Such vibrations may resonate in the engine's crankcase and be amplified by the same.